Why was Satan among the sons of God mentioned in Job Chapter 1 verse 6 and Chapter 2 verse 1?

Answer 1

Satan entered Jewish belief during what is known as the 'Persian period' as the loyal assistant of God, tasked with proving the righteousness of the faithful. The Book of Job is identified as having been written in the Persian period, partly because of its mention of Satan, but there are other indicators. Here, God is sending Satan "to and fro in the earth" seeking out those to test on behalf of God. The Book of Job is arguably an attempt to explain the existence of evil, in the presence of an all-powerful, benevolent God.

In later Christian belief, the existence of evil is explained through Satan as the evil adversary of God, but this has never formed part of Jewish belief.